As a result of search for antitumor substances, the present inventors found a novel antitumor substance BE-13793C (12,13-dihydro-1,11-dihydroxy-5H-indolo2,3-a!pyrrolo3,4-c!carbazole-5,7( 6H)-dione) among microbial metabolites, and disclosed it in a previous patent application (Japanese Laid-Open Patent No. 20277/'91) see The Journal of Antibiotics, Vol. 44, 20 pp. 723-728 (1991)!. Thereafter, they created a compound having more excellent antitumor activity by a chemical modification of BE-13793C, and disclosed it in another previous patent application (International Publication No. WO91/18003 and European Patent Laid-Open No. 0545195A1).
A problem to be solved by the present invention is to find a method which, in the preparation of indolopyrrolocarbazole derivatives having excellent antitumor activity as disclosed in the previous patent applications, can introduce a glycosyl group into such indolopyrrolocarbazole-derived antitumor substances smoothly and efficiently.